1. Field of the Invention
The present invention relates to an antenna matching switching system of a portable telephone for a TDMA system which is suitable for a mobile communication system.
2. Description of Related Art
In a conventional portable telephone for a TDMA system, electromagnetic wave is transmitted and received by one antenna such as a dipole antenna to realize a small size of apparatus. In the TDMA mobile communication system of this type, generally, because a frequency of a transmission signal (a transmission frequency) and a frequency of a reception signal (a reception frequency) are different from each other. Therefore, it is difficult to establish good antenna matching in both of the transmission frequency and the reception frequency using the same antenna matching circuit in case of the above antenna. A first conventional technique is disclosed in Japanese Laid Open Utility Model Application Disclosure (JP-U-Showa 59-147347) in which a low pass filter for preventing a high frequency transmission signal from being supplied to a reception side is provided between an antenna and the reception side.
In the TDMA communication system, the transmission period of the transmission signal in case of transmission and the reception period of the reception signal in case of the reception are different from each other. Therefore, a method of establishing the antenna matching by switching the matching state of the single antenna matching circuit between the transmission and the reception is disclosed in Japanese Laid Open Patent Disclosure (JP-A-Heisei 3-119836). In the disclosed second conventional example, the standing-wave ratio of an antenna is separately measured in the transmission and in the reception. Then, the optimization of the antenna matching in case of the transmission and the reception is attempted by feeding back this measuring results into the antenna matching circuit to adjust an antenna matching state. Note that the antenna matching is to optimize the electromagnetic wave radiation efficiency from the antenna in a predetermined frequency, i.e., to establish a maximum antenna gain in the predetermined frequency. A main factor is to achieve the impedance matching of the antenna.
In the above-mentioned conventional antenna matching technique, in order to transmit and receive the electromagnetic waves whose transmission frequency and reception frequency are different from each other, by the single antenna which is optimally matched, because many circuits are required such as an antenna characteristic measurement circuit, a control circuit of this measurement circuit, an antenna matching circuit in which parameters can be changed, and a parameter control circuit for the matching circuit. Therefore, the circuit structure becomes complicated and large size. In this manner, there is a problem that it is difficult to employ the circuit structure for the antenna matching circuit of a portable telephone in which it is strongly required to achieve a small size of apparatus.